Why can't the presenter engineer for themselves or have the producer do it?
Some of them do, though only the simpler programmes and news bulletins. But they they are employed for their journalism/editorial skills and it sounds much better with us engineering.
The majority of the programmes are a bit more complex than they could manage.
A lot of Radio 2, most of Radio 3, 4 and 5 have engineers doing the programmes.
BBC local stations don't have engineers to drive the programmes.